<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="initial-scale=1.0, maximum-scale=1.0, user-scalable=no" />
    <title>div模拟textarea文本域轻松实现高度自适应</title>
    <script src="http://code.jquery.com/jquery-2.2.4.js"></script>
    <style>
		#textarea {
		    display: block;
		    margin:0 auto;
		    overflow: hidden;
		    width: 550px;
		    font-size: 14px;
		    height: 18px;
		    line-height: 24px;
		    padding:2px;
		}
		textarea {
		    outline: 0 none;
		    border-color: rgba(82, 168, 236, 0.8);
		    box-shadow: inset 0 1px 3px rgba(0, 0, 0, 0.1), 0 0 8px rgba(82, 168, 236, 0.6);
		}
 
    </style>
</head>
<body>
	
	<textarea id="textarea" placeholder="回复内容"></textarea>
    <script>
        
		/**
		* 文本框根据输入内容自适应高度
		* @param                {HTMLElement}        输入框元素
		* @param                {Number}                设置光标与输入框保持的距离(默认0)
		* @param                {Number}                设置最大高度(可选)
		*/
		var autoTextarea = function (elem, extra, maxHeight) {
		        extra = extra || 0;
		        var isFirefox = !!document.getBoxObjectFor || 'mozInnerScreenX' in window,
		        isOpera = !!window.opera && !!window.opera.toString().indexOf('Opera'),
		                addEvent = function (type, callback) {
		                        elem.addEventListener ?
		                                elem.addEventListener(type, callback, false) :
		                                elem.attachEvent('on' + type, callback);
		                },
		                getStyle = elem.currentStyle ? function (name) {
		                        var val = elem.currentStyle[name];
		 
		                        if (name === 'height' && val.search(/px/i) !== 1) {
		                                var rect = elem.getBoundingClientRect();
		                                return rect.bottom - rect.top -
		                                        parseFloat(getStyle('paddingTop')) -
		                                        parseFloat(getStyle('paddingBottom')) + 'px';        
		                        };
		 
		                        return val;
		                } : function (name) {
		                                return getComputedStyle(elem, null)[name];
		                },
		                minHeight = parseFloat(getStyle('height'));
		 
		        elem.style.resize = 'none';
		 
		        var change = function () {
		                var scrollTop, height,
		                        padding = 0,
		                        style = elem.style;
		 
		                if (elem._length === elem.value.length) return;
		                elem._length = elem.value.length;
		 
		                if (!isFirefox && !isOpera) {
		                        padding = parseInt(getStyle('paddingTop')) + parseInt(getStyle('paddingBottom'));
		                };
		                scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
		 
		                elem.style.height = minHeight + 'px';
		                if (elem.scrollHeight > minHeight) {
		                        if (maxHeight && elem.scrollHeight > maxHeight) {
		                                height = maxHeight - padding;
		                                style.overflowY = 'auto';
		                        } else {
		                                height = elem.scrollHeight - padding;
		                                style.overflowY = 'hidden';
		                        };
		                        style.height = height + extra + 'px';
		                        scrollTop += parseInt(style.height) - elem.currHeight;
		                        document.body.scrollTop = scrollTop;
		                        document.documentElement.scrollTop = scrollTop;
		                        elem.currHeight = parseInt(style.height);
		                };
		        };
		 
		        addEvent('propertychange', change);
		        addEvent('input', change);
		        addEvent('focus', change);
		        change();
		};
		var text = document.getElementById("textarea");
        autoTextarea(text);// 调用
	</script>
</body>
</html>
